Magnetic deflection spectrographs are based on deflecting a narrow beam of ions in a magnetic field. In a given field, the angle of deflection varies by .sqroot.(1/mV), where V is the accelerating voltage. The deflected beam is received by a narrow slot located at a distance: the deflection angle is thus accurately defined, and for each value of V the slot selects ions having the corresponding mass m, where mV=C. The resolution naturally increases with the distance of the selector slot from the deflecting field. The drawbacks of this type of apparatus are obvious: high resolution requires considerable bulk. Further, the use of a narrow beam of ions greatly reduces the sensitivity of the system.